Update the account

I am a finchoice client,our agreement was for them to debit every last day of the month.Finchoice decided to debit on the 26 which was on Saturday.They short paid the payment themselves.only to find out on the 28 of March that they made a mistake.Dinchoixe does not want to accept their miatake they are putting a blame on me.On Tuesday I have made a transfer that was short,but my worry now is they have not updated the system as yet and they keep on calling and sending messages.i have called and send numerous emails with no success.It seems as if they are not interested to sort out my case.Pls help